Recruited Mayura fore this achievement
You need to complete "Supply line in the dessert" to unluck this mission.
The mission is called "Sasquatch Rampage" or http://translate.google.com/#en/ko/Sasquatch%20Rampage
The mission is very straight forward you need Begina to get close to here to have a convention with here after this she will desperate form the map. Take out the remaining enemies a She will join you after the mission is complete.
Warring you need this character to recute another character later she will be force to be in your party so don`t worry.